Matrix displays are well known in the art, where pixels are illuminated using matrix addressing as illustrated in FIG. 1. A typical display 100 comprises a plurality of picture or display elements (pixels) 160 that are arranged in rows and columns. The display incorporates a column data generator 110 and a row select generator 120. In operation, each row is sequentially activated via row line 130, where the corresponding pixels are activated using the corresponding column lines 140. In a passive matrix display, each row of pixels is illuminated sequentially one by one, whereas in an active matrix display, each row of pixels is first loaded with data sequentially. Namely, each row in the passive matrix display is only "active" for a fraction of the total frame time, whereas each row in the active matrix display can be set to be "active" for the entire total frame time.
With the proliferation in the use of portable displays, e.g., in a laptop computer, various display technologies have been employed, e.g., liquid crystal display (LCD) and light-emitting diode (LED) display. Generally, an important criticality in portable displays is the ability to conserve power, thereby extending the "on time" of a portable system that employs such display.
In a LCD, a backlight is on for the entire duration in which the display is in use. Namely, all pixels in a LCD are illuminated, where a "dark" pixel is achieved by causing a polarized layer to block the illumination through that pixel. In contrast, a LED display only illuminates those pixels that are activated, thereby conserving power by not having to illuminate dark pixels.
FIG. 2 illustrates a prior art active matrix LED pixel structure 200 having two NMOS transistors N1 and N2. In such pixel structure, the data (a voltage) is initially stored in the capacitor C by activating transistor N1 and then activating "drive transistor" N2 to illuminate the LED. Although a display that employs the pixel structure 200 can reduce power consumption, such pixel structure exhibits nonuniformity in intensity level arising from several sources.
First, it has been observed that the brightness of the LED is proportional to the current passing through the LED. With use, the threshold voltage of the "drive transistor" N2 may drift, thereby causing a change in the current passing through the LED. This varying current contributes to the nonuniformity in the intensity of the display.
Second, another contribution to the nonuniformity in intensity of the display can be found in the manufacturing of the "drive transistor" N2. In some cases, the "drive transistor" N2 is manufactured from a material that is difficult to ensure initial threshold voltage uniformity of the transistors such that variations exist from pixel to pixel.
Third, LED electrical parameters may also exhibit nonuniformity. For example, it is expected that OLED (organic light-emitting diode) turn-on voltages may increase under bias-temperature stress conditions.
Therefore, a need exists in the art for a pixel structure and concomitant method that reduces current nonuniformities due to threshold voltage variations in a "drive transistor" of the pixel structure.